In a variety of mechanical applications it is often required that one element of a machine be moved longitudinally with respect to another element of the machine along a threaded leadscrew with repeatable accuracy. One example of such an application is a print head which must be moved with respect to a platen on a data printer on repetitive occasions with precision.
One manner of providing such a motion control device is to attach a moveable machine element to a nut on the leadscrew. When the leadscrew is rotated, its rotational movement is translated to longitudinal movement of the nut and the attached moveable element along the longitudinal axis of the leadscrew. The position of the nut may be adjusted along the longitudinal axis of the leadscrew to determine the range of longitudinal movement of the element attached thereto.
Providing a nut which is adjustable with respect to the leadscrew, however, introduces the problem of backlash. Backlash is the play or slop which results from loose connections between mechanical elements, such as nuts and leadscrews. Because backlash introduces longitudinal positioning errors, elimination of backlash in precision motion control devices is desirable.
Mechanisms for reducing backlash in nut and leadscrew assemblies are known. One manner of reducing backlash is to separate a nut on a leadscrew into two parts and install a compressed spring between the two parts of the nut. The compressed spring applies a biasing force, which is exerted in opposite longitudinal directions along the axis of the leadscrew, to the surfaces of both parts of the nut to which it is coupled. The biasing force urges the internal threads on both portions of the nut into contact with external threads on the leadscrew to thereby reduce backlash. Many prior art anti-backlash nut assemblies disclose two-piece nut portions which are separated by a spring to force the internal threads on at least one of the two pieces of the nut into contact with external threads on the leadscrew to reduce backlash.
One disadvantage of prior art anti-backlash nut assemblies is they generally do not have means for adjusting the compressive force or preload provided by the spring intermediate the two portions of the nut, and especially when the nut assembly is installed on the leadscrew. The compressive force provided by the spring must be adjustable to accommodate light or heavy tension applications. Moreover, adjustability of the compressive force of the spring without removing the nut assembly from the leadscrew is important if it is desired to maintain the exact position of the nut assembly on the leadscrew for continued operation.
Another disadvantage of prior art anti-backlash nut assemblies is that they generally have an externally mounted spring which results in a shortened life of the assembly due to exposure of the spring and the retaining mechanism to dirt or corrosive contaminants, as well as contributing to a loss of spring lubricant. In addition, due to the external mounting of the spring, a mounting flange or other means must be utilized to mount any external assemblies such as a print head of a printer. Also, the external spring mount design requires a larger overall size of the nut assembly. Further, the exposed spring may be a "pinch point" or safety issue. Still further yet, prior art external spring mount designs do not capture broken pieces should the spring fail, which could result in serious damage to external machinery. Last, prior art external spring mount designs may result in a change of preload during load reversals.
Accordingly, it is highly desired to provide an improved anti-backlash nut and spring assembly having adjustment means to adjust the spring preload while the assembly is installed on a leadscrew while preventing a change in the amount of preload during load reversals. It is also desired to provide such a nut and spring assembly wherein the spring is internally mounted within a housing to avoid exposure to contaminants, retain necessary lubricants, and provide an added safety feature whereby extraneous matter may be caught in the spring. It is also desired to provide an anti-backlash nut assembly with a reduced number of parts wherein additional mounting flanges or other mounting parts are not needed. Still further yet, it is desired to provide a design which captures broken pieces should the spring fail, and thus preventing serious damage to external machinery.